Preheat a grill pan over medium-high heat. Toss the apple slices with 2 teaspoons olive oil and cook, turning occasionally, until marked and tender, 3 to 4 minutes.
Lay the tortillas on a cutting board. Top each with one-quarter each of the grilled apple and brie on one side, then fold the tortillas in half to cover the filling.
Heat the remaining 2 teaspoons olive oil in a large nonstick skillet over medium heat. Working in batches, cook the quesadillas until golden brown and the cheese melts, about 2 minutes per side. Meanwhile, mix the mustard and maple syrup in a bowl. Slice each quesadilla into wedges. Drizzle with the maple-mustard syrup and top with the bacon.
Photograph by Kat Teutsch